Tin electrodeposition process



United States Patent 3,346,468 TIN ELECTRODEPOSITION PROCESS Jan C. Jongkind, Roseville, Mich., assignor to M & T Chemicals Inc, New York, N.Y., a corporation of Delaware No Drawing. Filed May 8, 1964, Ser. No. 366,146 5 Claims. (Cl. 204-54) ABSTRACT OF THE DISCLOSURE In accordance with certain aspects of this invention an alkaline tin oxide s-o'l characterized by its substantially complete convertibility to stannate when in contact with solutions containing 5-100 g./l. of potassium hydroxide at temperature of 50 C.100 C. may be prepared by the process which comprises reacting at less than 75 C. an alkali metal stannate in an aqueous solution with acid to a final pH of less than about 6 thereby precipitating hydrous stannic oxide, separating said hydrous stannic oxide from said aqueous medium, washing said hydrous stannic oxide thereby removing water-soluble ions, peptizing said hydrous stannic oxide with peptizing agent selected from the group consisting of potassium hydroxide and potassium stannate, thereby forming a final solution, maintaining the molar ratio of potassium to tin in the final solution at 0.1-1.5, and maintaining said hydrous stannic oxide at temperature below 75 C. prior to said peptizing.

This invention relates to a novel process for electroplating of tin. More specifically it relates to a novel technique for replenishing the tin content of a tin-plating bath.

As is well known to those skilled in the art, tin may be electroplated onto Various basis metal cathodes from electrolytic baths containing alkali metal stannate, preferably potassium stannate, and alkali metal hydroxide, preferably potassium hydroxide. As plating continues, tin is removed from the bath. The tin content of the bath may be restored continuously by the use of a soluble tin anode. Use of ,a soluble anode system is disadvantageous in that is requires operation within a rather limited range of anode current density. Operation outside of this limited range may yield either a rough dark plate or alternatively inactivation of the anode with resulting failure of the anode to replenish the tin depleted from the bath, this being accompanied by an undesirable increase in concentration of alkali metal hydroxide. Many practical plating operations may require operation outside the narrow limits of anode current density required by the use of soluble anodes, and in such systems, it may not be advantageous to use a soluble tin anode.

Accordingly, it has been common to use inert anodes typically stainless steel anodes, and to attempt to replenish the depleted tin by the addition to the bath of tin compounds. Typically alkali metal stannate, e.g. potassium stannate, may be added; but this is highly disadvantageous in that it adds alkali metal ion to the bath and this ultimately may build up the concentration thereof to a point at which no more alkali metal stannate ,will dissolve, at

which point the bath must be discarded. Another disadvantage may be that the concentration of alkali metal hydroxide increases and this must be corrected by neutralization with acid, preferably acetic acid with the very real danger of over-neutralization and sludging.

It is an object of this invention to provide a process for electrodeposition of tin. It is a further object of this invention to provide a process for replenishing the tin content of an alkaline stannate tin-plating bath. It is a further object of this invention to provide a novel composition which may be added to alkaline tin-plating baths to replenish the tin content. Other objects will be apparent to those skilled in the art from inspection of the following description.

In accordance with certain aspects of this invention an alkaline tin oxide sol characterized by its substantially complete convertibility to stannate when in contact with solutions containing 5-100 g./l. of potassium hydroxide at temperature of 50 C.100 C. may be prepared by the process which comprises reacting at less than C. an alkali metal stannate in an aqueous solution with acid to a final pH of less than about 6 thereby precipitating hydrous stannic oxide, separating said hydrous stannic oxide from said aqueous medium, washing said hydrous stannic oxide thereby removing water-soluble ions, peptizing said hydrous stannic oxide with peptizing agent selected from the group consisting of potassium hydroxide and potassium stannate thereby forming a final solution, maintaining the molar ratio of potassium to tin in the final solution at 0.1-1.5, and maintaining said hydrous stannic oxide at temperature below 75 C. prior to said peptizin-g.

The novel alkaline tin oxide sol of this invention may be prepared by the reaction of alkali metal stannate, preferably potassium stannate or sodium stannate, with acid typically as follows:

In practice of this process, ,a body of alkali metal stannate, sodium stannate or potassium stannate, typically sodium stannate may be employed containing 10-500 g. l., typically 50 g./l. To this solution at temperature below 75 C. and preferably 35 C.-70 0., preferably 50 C. there may be added acid eg, acetic acid, hydrochloric acid, nitric acid, sodium bicarbonate, etc. preferably dilute (e.g. 10%) sulfuric acid. Preferably the acid may be added slowly; when sulfuric .acid is used, control may be effected to keep the temperature in the noted range. The acid may be added in amount of about two equivalent thereof per equivalent amount of tin; the pH during the addition may decrease from a pH greater than 12 down to a final pH of less than about 6 and preferably 2.56, most preferably to about pH 4.6. As this occurs, the hydrous stannic oxide precipitates in the form of a flocculent white mass.

If the body of alkali metal stannate is a crude liquor, as obtained from a detinning operation, and particularly if it contains organic impurities such as degraded lacquers, etc., it may be preferred to use sodium bicarbonate as the acid and to achieve partial neutralization to pH of about 8. The sodium bicarbonate may be added to the alkali metal stannate solution or may be generated in situ, e.g. by bubbling CO into a solution of sodium stannate. Other equivalent acids may be employed to lower the pH to about 8. At this pH, the hydrous stannic oxide may precipitate and after separation from supernatant liquor, may be readily washed free of impurities. The so-washed precipitate may then be reslurried in an aqueous medium and treated with acid, preferably acetic acid to the final pH of less than about 6 and preferably 2.56, most pref erably to about pH 4.6.

The so-precipitated hydrous tin oxide may be separated from supernatant aqueous medium as by decantation, filtration, centrifuging, etc-preferably by decantation. The precipitate may be Washed by mixing with water, and again separating. Preferably, washing may be done 4-6 times. At the conclusion of washing the precipitate may be substantially free of (a) sodium ions which may have been present, e.g. if the charge material was sodium stannate; and (b) anions including, e.g. sulfate, which may have been introduced as from the precipitating acid. The total content of water-soluble ions in the precipitate may normally be less than about 0.2%. In the preferred embodiment, five decantation steps may be employed followed by filtration. The precipitate may be found to have a tin content of 10%50% on a wet basis. It is preferred to filter to a tin content of 28%40% by weight on a wet basis. At this point, it may be prefered to adjust the tin content to a predetermined level, depending on the amount of tin desired in the final sol. Typically, it may be desirable to adjust the tin content of the precipitate to about 28%. To this slurry, there may then be added a peptizing agent selected from the group consisting of potassium hydroxide and potassium stannate, in amount sufficient to form a colloidal solution of the precipitate and to effect peptization. The peptizing agent may be added in amount sufiicient to permit attainment typically of about 30% tin in the final solution and of a ratio of potassium to tin of 0.1-1.5. Preferably, the peptizing agent may be added as a solid, with agitation, during which time the temperature may be below about 75 C. and preferably 20 C.- 30 C., typically room temperature.

The colloidal solution so prepared may be characterized by its tin content of 10%50%, preferably 30%, and by its ratio of potassium ion to tin ion of 0.11.5, preferably 0.3. It is a particular property of this solution that it may be stable for an indefinite period of time at temperature of 50 C. or less, typically at room temperature. It is a further characteristic that the novel alkaline tin oxide sol may readily be dispersed to give a clear, stable dispersion when contacted with solutions containing 5-100 g./l. of potassium hydroxide at a temperature of 50100 C. The resultant dispersion may be maintained in the noted temperature range, whereupon the tin oxide in the sol may be converted to stannate. Typically, 100% conversion to stannate may be realized in a short time, typically four hours or less. It is a particular advantage of this process that the novel alkaline tin oxide sol will retain its ready convertibility to stannate after prolonged storage, in contrast with stannic oxide hydrate paste which is not completely convertible to stannate after storing.

It is a further feature of the novel solution of this invention that, when viewed under an electron miscroscope, it appears to contain a plurality of highly crystallinecrystallite stannic oxide particles each of size less than angstrom units. When the solution is permitted to dry in air, the dry particles appear to form an interlocking network of particle chains each having a thickness of about 4-8 particles and a length of about 10-50 particles.

According to certain of its aspects the novel tin plating process of this invention may comprise electrodepositing tin from an alkali metal stannate bath containing an insoluble anode (or a low efficiency soluble anode) and a cathode thereby depleting the tin content of said bath and replenishing the tin content of said bath by adding an alkaline tin oxide sol containing crystalline-crystallite stannic oxide.

Electroplating of tin in practice of this invention may be effected from an aqueous bath containing alkali metal stannate, preferably potassium stannate in amount of g./l.450 g./l., preferably 100 g./l. and alkali metal hydroxide, preferably potassium hydroxide in amount of 7.5 g./l.35 g./l., preferably 2.25 g./l. The anode may preferably be an insoluble anode e.g. stainless steel, etc. The cathode may be any metal on which an electrolytic tin plate is desired e.g. steel, brass, copper, etc.

During electroplating, the bath may typically be maintained at 60 C.95 C., preferably 80 C. The cathode current density may be up to 10 amperes per square decimeter (a.s.d.) and typically about 6 a.s.d. over a plating time of 5-30 minutes, typically minutes. The anodic current density may be several times higher than the cathodic current densities. Typically the anodic current density may be as high as 50 a.s.d. This unexpected feature of the invention facilitates plating on the inside of pipes, couplings, rings where the geometry of the article is such that the anode must, because of the geometry, be considerably smaller than the cathode.

As the bath is used over a period of time, for every 118.7 grams of tin plated out, 112 grams of potassium hydroxide (in the case of the potassium bath) or grams of sodium hydroxide (in the case of the sodium bath) is generated as by the following equation:

If no corrective action be taken, the bath becomes unbalanced, the tin content drops, and in due course no more tin will plate out. The content of free hydroxide rises and the cathode current efficiency drops clue to this factor alone.

In practice of this invention, there may be added to the electroplating bath the alkaline tin oxide sol hereinbefore disclosed. The addition of the colloidal solution may be determined by the ampere hours for which the bath is used or by measuring the tin content of the bath or e.g. the potassium hydroxide content of the bath. Preferably the colloidal solution may be added to the bath at a rate of 1.107 grams of tin per ampere hour of plating. In the preferred embodiment wherein the colloidal solution contains 30% tin, 3.69 grams of solution may be added per ampere hour.

If control be efiected by measuring the tin content of 'the bath, then for example 3.3 grams of 30% tin oxide sol may be added for each one g./l. of tin lost, thus compensating for the tin plated out.

It will be apparent that the addition may be continuous or incremental. When incremental, it preferably may be done at intervals sufiicient to prevent the free e.g. potassium hydroxide from rising above or preferably closely approaching 30 g./l. In a typical industrial operation using currents of the order of 0.25-0.50 amperes per liter, then the free potassium hydroxide content may rise by 0.28-0.56 g./l. per hour; and this may be corrected by the addition of 0.91.8 grams of the preferred 30% tin sol per liter per hour.

Use of this novel composition permits tin plating to be carried on indefinitely from a tin bath with no deterioration of the quality of the plate.

It is a particular feature of the process of this invention that addition of the colloidal solution permits maintenance of the bath with no undesirable excessive build-up of potassium ion.

If replenishment of tin were made by addition of potassium stannate, the excess potassium hydroxide may have to be neutralized by acid e.g. acetic acid (as is commonly done), then the potassium content of the bath may rise to such an extent that after about 375 ampere hours per liter have passed through the bath, no more potassium stannate can be dissolved in the bath because of the presence of the excess of potassium ions; the bath may have to be discarded as unuseable.

It is also a particular feature of the novel product of this invention that when added to tin plating baths as herein noted, it goes into solution as the desired stannate ion.

Practice of this invention may be observed from the following examples:

Example 1 A solution of 1000 grams of potassium stannate dissolved in 2 liters of water was heated to 49 C. and 700 grams of sodium bicarbonate was added thereto in small increments. During the addition, the solution was stirred and the temperature was maintained at 49 C.i3 C. The precipitate which formed was filtered and washed with 500 ml'. of cold water. After washing, the precipitate was suspended in 1.5 liters of water and ml. of glacial acetic acid was added to bring the pH to about 5. The precipitate was again filtered and washed with water. It was then mixed with 200 grams of solid potassium stannate, whereupon a fluid clear, slightly amber alkaline 75 tin oxide sol was obtained. The product sol weighed 1465 grams, had a specific gravity of 1.63, and contained 28.6% by weight tin.

Example 2 50 ml. of an aqueous potassium hydroxide solution containing 240 grams KOH per liter was transferred to a 100 ml. volumetric flask. An amount of the alkaline tin oxide sol of Example 1 equivalent to 11.5 grams of tin (41 grams of the sol) was added thereto and the Example 3 A tin plating solution was made up by dissolving potassium stannate and potassium hydroxide in water to give 283 grams per liter of K Sn(OH) and 20 grams per liter of free KOH. The bath was heated to about 77 C. and electrolyzed at a cathode current density of 6.5 amperes per square decimeter using stainless steel anodes and steel cathodes. The alkaline tin oxide sol of Example 1 was added to the bath at the rate of 3.9 grams of sol for each ampere-hour of operation to replenish the tin depleted by plating. The bath was operated and maintained in this manner for 123 ampere-hours per liter. Throughout the test, the in electroplate obtained was highly satisfactory and had a light grey, satin, smooth appearance. At the end of the test, the bath was analyzed and found to contain 350 grams per liter of K Sn(OH) and 24 grams per liter of free KOH. From this example, it may be seen that the novel alkaline tin oxide sols of this invention are outstanding sources of tin for the replenishment of tin plating baths; that the tin plate obtained was highly satisfactory; and that the use of the novel products of this invention substantially eliminates the problem of build-up of free KOH in the plating bath.

Example 4 For purposes of comparison, the procedure of Example 1 was repeated, except that the temperature was not maintained within the limits of this invention as hereinbefore set forth.

Specifically, a solution of 1000 grams of potassium stannate dissolved in 2 liters of water was heated to 82 C. and 700 grams of sodium bicarbonate was added thereto in small increments. During the addition, the solution was stirred and the temperature was maintained at 82 C: 3 C. The precipitate which formed Was filtered and washed with 500 ml. of cold water. After washing, the precipitate was suspended in 1.5 liters of water and 120 ml. of glacial acetic acid was added to bring the pH to about 5. The precipitate was again filtered and washed with water. It was then mixed with 200 grams of potassium stannate, whereupon there was obtained a viscous, murky product containing numerous undissolved particles which were not dissolved after 24 hours.

When the product of this example was subjected to the solubility test described in Example 2, it gave a dense white precipitate which was not dissolved even after prolonged agitation.

Example 5 1,000 liters of a solution obtained from a detinning operation was analyzed and found to contain 49.5 grams per liter of tin and 3 grams per liter of free sodium hydroxide. Due to the presence of decomposition products from lacquers contained on the scrap, the solution was black and contaminated with organic residues. The solution was heated to 51 C. and 90.7 kilograms of sodium bicarbonate was added incrementally with stirring while the temperature was maintained at about 50 C. The precipitate which formed was allowed to settle overnight and the supernatent liquor was decanted. The precipitate was transferred to a filter and washed with water until white. The washed precipitate was suspended in liters of water and 10- liters of glacial acetic acid was added thereto to bring the pH to 4.6. The precipitate was again filtered and washed with water to a neutral filtrate. The residue after washing weighed 119.3 kilograms and contained 42.89% tin.

To this residue was added 63 liters of water and the mixture was stirred. 45.4 kilograms of potassium stannate was added with stirring at room temperature, whereupon there was obtained 227.7 kilograms of a clear, fluid slightly greenish, alkaline tin oxide sol having a specific gravity of 1.66 and a tin content of 30% by weight. This sol was treated with carbon to remove the color.

When this product was subjected to the solubility test described in Example 2, it produced a clear, stable solution with no evidence of insoluble material.

When it was employed as the tin-replenishing agent in a plating operation as describd in Example 3, it was found that the bath was still producing highly satisfactory tin plate after 430 ampere-hours per liter of operation and that the free potassium hydroxide content of the bath remained essentially constant throughout the test.

Example 6 To a solution of 725 .7 kilograms of potassium stannate in 4,542.4 liters of water was added a solution of 18 parts by volume concentrated sulfuric acid and 82 parts by volume water until a pH of 3.0 was reached. The temperature during the addition was maintained below 35 C.; the precipitate was washed with water to a neutral filtrate. The Washed precipitate was mixed with 215 kilograms of potassium stannate at room temperature, whereupon a clear, fluid alkaline tin oxide s01 containing grams per liter of tin was formed.

When this sol was subjected to the solubility test described in Example 2, it produced a clear, stable solution with no evidence of insoluble material.

Example 7 To a solution of 500 grams potassium stannate in 1200 ml. of water was added 410 m1. of a solution of one part concentrated sulfuric acid in 4 parts of water to give a pH of 3.85. During the addition, the solution was stirred and maintained at 3238 C. An additional 1000 ml. of water was added and the precipitate formed during the addition was filtered and washed to a neutral filtrate. The washed precipitate was mixed with 50 grams of potassium hydroxide whereupon there was formed 1800 ml. of fluid, clear alkaline tin oxide sol containing 108 grams of tin per liter.

When subjected to the solubility test of Example 2, the sol of this example produced a clear, stable solution with no evidence of insoluble material.

Although this invention has been illustrated by reference to specific examples, numerous changes and modifications thereof which clearly fall within the scope of the invention will be apparent to those skilled in the art. 7

I claim:

1. The novel tin plating process which comprises electrodepositing tin from an alkali metal stannate aqueous bath containing an insoluble anode and a cathode thereby depleting the tin content of said bath, and replenishing the tin content of said bath by adding thereto an alkaline tin oxide sol prepared by the process which comprises reacting at less than 75 C. an alkali metal stannate in aqueous solution with acid to a final pH of less than about 6 thereby precipitating hydrous stannic oxide, separating said hydrous stannic oxide from said aqueous solution, washing said hydrous stannic oxide thereby removing water-soluble ions, peptizing said hydrous stannic oxide with peptizing agent selected from the group consisting of potassium hydroxide and potassium stannate thereby forming a final solution, maintaining the molar ratio of. potassium to tin in the final solution at 0.1-1.5, and maintaining said hydrous stannic oxide at temperature below 75 C. prior to said peptizing.

2. The novel tin plating process which comprises electrodepositing tin from an alkali metal stannate aqueous bath containing an insoluble anode and a cathode thereby depleting the tin content of said bath, and replenishing the tin content of said bath by adding thereto an alkaline tin oxide sol prepared by the process which comprises reacting at less than 75 C. an alkali metal stannate in aqueous solution with acid to a pH of about 8 thereby precipitating hydrous stannic oxide, separating said hydrous stannic oxide from said aqueous solution, washing said hydrous stannic oxide thereby removing water-solu ble impurities, reslurrying the so-washed precipitate in aqueous medium, reacting the so-formed slurry with acid to a final pH of less than about 6, separating said hydrous stannic oxide from said aqueous medium, Washing said hydrous stannic oxide thereby removing water-soluble ions, peptizing said hydrous stannic oxide with peptizing agent selected from the group consisting of potassium hydroxide and potassium stannate thereby forming a final solution, maintaining the molar ratio of potassium to tin in the final solution at 0.1-1.5, and maintaining said hydrous stannic oxide at temperature below 75 C. prior to said peptizing.

3. The novel tin plating process which comprises electrodepositing tin from an alkali metal stannate aqueous bath containing an insoluble anode and a cathode thereby depleting the tin content of said bath, and replenishing the tin content of said bath by adding thereto an alkaline tin oxide sol prepared by the process which comprises reacting at -70 C. a 10-500 g./1, aqueous solution of alkali metal stannate with acid to a final pH of 2.5-6 thereby precipitating hydrous staunic oxide, separating said hydrous stannic oxide from said aqueous solution, washing said hydrous stannic oxide thereby removing water-soluble ions, adjusting the tin content of said hydrous stannic oxide to 28-40% by weight, adding to said hydrous stannic oxide peptizing agent selected from the group consisting of potassium hydroxide and potassium stannate thereby peptizing said hydrous stannic oxide and thereby forming a final solution, maintaining the molar ratio of potassium to tin in the final solution at 0.1-1.5, and maintaining said hydrous stannic oxide at temperature 35-7 0 C. prior to said peptizing.

4. The process claimed in claim 1 wherein said bath is maintained at -95 C.

5. The process claimed in claim 1 wherein said novel alkaline tin oxide sol is added to said bath at a rate of about 1.107 grams of tin per ampere hour of plating.

OTHER REFERENCES Weiser, Harry Boyer, The Hydrous Oxides, pp. 215- 225, 1926.

JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

G. KAPLAN, Assistant Examiner. 

1. THE NOVEL TIN PLATING PROCESS WHICH COMPRISES ELECTRODEPOSITING TIN FROM AN ALKALI METAL STANNATE AQUEOUS BATH CONTAINING AN INSOLUBLE ANODE AND A CATHODE THEREBY DEPLETING THE TIN CONTENT OF SAID BATH, AND REPLENISHING THE TIN CONTENT OF SAID BATH BY ADDING THERETO AN ALKALINE TIN OXIDE SOL PREPARED BY THE PROCESS WHICH COMPRISES REACTING AT LESS THAN 75*C. AN ALKALI METAL STANNATE IN AQUEOUS SOLUTION WITH ACID TO A FINAL PH OF LESS THAN ABOUT 6 THEREBY PRECIPITATING HYDROUS STANNIC OXIDE, SEPARATING SAID HYDROUS STANNIC OXIDE FROM SAID AQUEOUS SOLUTION, WASHING SAID HYDROUS STANNIC OXIDE THEREBY REMOVING WATER-SOLUBLE IONS, PEPTIZING SAID HYDROUS STANNIC OXIDE WITH PEPTIZING AGENT SELECTED FROM THE GORUP CONSISTING OF POTASSIUM HYDROXIDE AND POTASSIUM STANNATE THEREBY FORMING A FINAL SOLUTION, MAINTAINING THE MOLAR RATIO OF POTASSIUM TO TIN IN THE FINAL SOLUTION AT 0.1-1.5, AND MAINTAINING SAID HYDROUS STANNIC OXIDE AT TEMPERATURE BELOW 75*C. PRIOR TO SAID PEPTIZING. 